Froth and method of producing froth

ABSTRACT

Foam for making pads and belts with controlled, reproducible microcellular structure and method of making such foam in a fast and efficient manner. Under constant pressure and temperature, a prepolymer is mixed with the nucleation surfactant in a tank in the presence of a frothing agent metered into the tank by way of a dip tube or sparge. The nitrogen gas is sheared into small bubbles and is drawn off from the headspace of the tank creating a continuous flow of nitrogen. The pressure of the tank may vary from any absolute pressure down to near complete vacuum, thereby all but eliminating the pressure requirement. The froth of the present invention has a more consistent cell structure and increased cell count.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of a co-pending U.S. patentapplication Ser. No. 09/317,973, entitled “Foam Semiconductor PolishingBelts and Pads,” filed May 25, 1999, which is a nonprovisionalapplication claiming priority to U.S. Provisional Application No.60/087,740, filed Jun. 2, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved foam and method ofproducing such foam for a variety of applications, includingsemiconductor polishing pads and belts.

2. Description of the Related Art

Silicon wafers are produced as precursors from which microelectronicsemiconductor components are produced. The wafers are sliced or cut fromcylindrical silicon crystals, parallel to their major surfaces, toproduce thin disks, typically 20-30 centimeters in diameter, althoughlarger or smaller wafers are possible. The resulting wafers must bepolished to give flat and planar surfaces for proper formation ofelectronic components to form integrated chip semiconductor devices.Typically, a 20-cm diameter wafer will produce 100 or moremicroprocessor chips.

The design size of such integrated chips is steadily decreasing, whilethe number of layers applied, e.g., by various sequences of depositing,patterning, and etching of features onto the silicon surface, is rising.Present semiconductors typically incorporate up to 8 or more metallayers, and it is expected that future designs will contain even morelayers. The decrease in the size of the circuitry and the increase inthe number of layers applied are leading to even more stringentrequirements on the smoothness and planarity of the silicon andsemiconductor wafers throughout the chip manufacturing process, sinceuneven surfaces may undermine the patterning process and the generalintegrity of the resulting circuit.

In the semiconductor chip fabrication process, it is well-known thatthere is a need to polish a semiconductor wafer. This polishing istypically-accomplished by a chemical mechanical process (CMP). Onestandard CMP wafer polishing technique is to position a wafer over arotating polishing pad that is usually disk-shaped, and is mounted on alarge turntable. A chemical-mechanical polishing slurry is usuallyapplied to the surface of the pad, and the wafer is held in place by anoverhead wafer carrier while being polished by the rotating pad andslurry. The slurry is generally made up of an aqueous solution withmetallic or non-metallic particles such as, for example, aluminum orsilica abrasives that create the added friction for the polishingprocess.

A significantly different approach is the so-called Linear PlanarizationTechnology (LPT), wherein the polishing pad is mounted onto a supportingbelt and is used to polish the wafer, in place of the flat turntableform of the polishing tool. The belt used in this method is described inEP-A-0696495 and comprises an endless sheet of steel or other highstrength material, having a conventional flat polyurethane polishing padaffixed to it with adhesive. As with the rotating pad, the pad used forLPT CMP polishing receives a chemical-mechanical polishing slurry thatis distributed over the surface of the belt.

State of the art semiconductor polishing pads are made from high densitypolyurethane foams that have a functional porous structure, which aidsthe distribution of the chemical-mechanical polishing slurry and reduceshydroplaning, for example. Such pads are formed from a polymericcomposition that comprise a dispersion of thin-walled, hollow plasticbeads or “microspheres,” which can potentially provide a controlled andconsistent microcellular structure.

However, there are some limitations to the use of hollow microspheres inpolishing pads. The size and shape of the foam's cells are restricted tothe limited sizes and shapes of the commercially available microspheres.In addition, microspheres may be too abrasive for some delicatepolishing operations, e.g. certain steps in semiconductor manufacturingincluding, but not limited to, chemical mechanical polishing of softmetal layers. Typically, the microspheres are extremely light weight andflammable, posing significant material handling difficulties, includingdust explosion hazards. The light-weight microspheres are also difficultto disperse in the polyurethane resins. They tend to clump and foulprocess equipment, and often entrain significant amounts of air, whichleads to problematic variations in porosity of the cured foam. Also, themicrospheres can distort, collapse, or melt if processed at hightemperatures that are routinely used in processing polyurethanes andother potential pad materials.

Foam density, a measurement of the mass of froth per unit volume, is oneof the most important properties of froth, directly affecting thedurability and support of the foam. It is commonly measured andexpressed in pounds per cubic foot (pcf) or kilograms per cubic meter(kg/m³), but may also be stated as g/cc. Foam density is directlyrelated to the specific gravity of the foam.

Although there are several conventional ways to create high densitypolyurethane foam, including mechanical frothing and chemical blowingprocesses, pads produced by the conventional methods have not beensuccessful in semiconductor polishing. While the polishing pads producedby the conventional method may be suitable for polishing glass and otherlow technology applications, they have not been as successful insemiconductor polishing, which is a more precise and more delicateapplication, because of the variability in pad cell structure and padproperties. Often times, the density of the foam produced by theseconventional frothing methods varies greatly due to the conventionalmethods' inability to consistently produce foam within a preferablerange of specific gravities or because of impurities, e.g. oxygen,contained within the foam. Another significant drawback of the prior artfrothing methods is the time involved in producing the froth.Consequently, there is a need for a time efficient method ofmanufacturing froth falling within an optimal specific gravity range.

OBJECTS OF THE INVENTION

A method of producing froth used to produce a microcellular polishingpad or belt in a more expedient and efficient manner than theconventional prior art methods.

A method of producing froth used to produce a microcellular polishingpad or belt in which the froth produced has greater cell density thanthe froth produced using the conventional prior art methods.

A method of producing froth used to produce a microcellular polishingpad or belt which allows for better control, i.e., tighter range, overthe specific gravity of the resulting froth.

A method of producing froth used to produce a microcellular polishingpad or belt which minimizes the volatile components and oxygencontamination in the froth.

A method of producing froth used to produce a microcellular polishingpad or belt wherein the froth density and average cell size can bevaried independently of each other.

Another object of the invention is to provide a froth with a greatercell density and more uniform cell structure than the prior art froth.

Another object of the invention is to provide a froth having a preferredspecific gravity.

Another object of the invention is to provide a froth that isessentially free of volatile components and contaminants.

Other aspects and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the principle ofthe invention.

SUMMARY OF THE INVENTION

This invention comprises high density foam semiconductor polishing padsand belts with controlled, reproducible microcellular structures thathave been produced by a novel method of mechanical frothing. Thisinvention provides an improved method for producing semiconductorpolishing pads with consistent cell structure and properties, thatperform equal to or better than the state-of-the-art polishing pads. Themethod also provides increased degrees of freedom and convenience inproducing pads with different desired cell structures.

This invention also comprises a novel method of mechanically frothingthe prepolymer material used for making foam semiconductor polishingpads and belts with controlled, reproducible microcellular structure.This invention provides a fast and efficient method for producing frothwhile also providing improved control over the specific gravity of theresulting froth. By providing more control over the specific gravity ofthe froth as well as minimizing the volatile components and contaminantsthat are contained within the froth, the present invention provides apad or belt with a more consistent cell structure and greater celldensity than the state-of-the-art pads. Due to the increase in celldensity and a more consistent cell structure, the pads and belts of thepresent invention allow for increased polishing rates and more evenplanarization of the workpiece. In addition, the present invention alsoprovides increased degrees of freedom and convenience with respect tothe pressure under which the resulting froth and pads are produced.

It should be noted at this point that the term “pad,” as used herein,refers to polishing disks, polishing belts and any other geometric shapethat may serve to polish semiconductor wafers. As a result, the term“pad” may be used interchangeably with the term “belt.” Moreover, theterm “polishing disk” refers generally to any polishing pad that is usedon a rotating, moving or stationary platen, regardless of the pad'sshape. In other words, even though most polishing pads used on rotatingplatens are in fact disk-shaped, the term “polishing disk,” as usedherein, is not confined to that shape.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the objects of the presentinvention, the Detailed Description of the Invention will be taken withthe drawings, wherein:

FIG. 1 is block diagram showing the production flow of the method of thepresent invention.

FIG. 2 is a perspective view of a tank with agitator used in performingthe methods of the claimed invention.

FIG. 3 is an axial view of a 4-blade impeller used in performing themethods of the claimed invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention comprises a method for making high density foamsemiconductor polishing pads and belts with controlled, reproduciblemicrocellular structure by mechanical frothing. This invention alsocomprises foam and pads with increased cell density and uniformdistribution of cells having a preferred specific gravity. Preferably,the specific gravity of the foam is within the range of 0.7-1.0 g/cc.More preferably, the specific gravity of the foam is within the range of0.80-0.95 g/cc. The preferable range of specific gravity of the pad is05-1.2 g/cc. Preferably, the specific gravity for the pad is 0.7-1.0g/cc. Even more preferable, the specific gravity for the pad made fromthe foam of the present invention is 0.85-0.95 g/cc.

The method of the present invention involves agitating a liquid polymerresin at a controlled temperature and pressure in order to produce astable froth. Next, the resin froth is metered into a mix head where itis typically combined with the desired amount of curative before beinginjected or poured into a mold.

The resin material is typically polyurethane but can be any suitablethermoset polymeric material. In the case of urethanes, any suitableformulation is acceptable, including the incorporation or utilization ofvarious fillers, catalysts, additives, and surfactants. Catalysts andblowing agents can be used to create an open-celled structure in thepolishing pad or to enlarge the cells after the mixture is poured intothe mold. It has been found that nucleation surfactants, that arecommonly used in the manufacture of low density blow foams, are usefulfor producing a stable froth, which is critical to the presentinvention. One particularly useful nucleating surfactant is a blockpolymer containing at least one block comprising polydimethylsiloxaneand at least one other block comprising polyether, polyester, polyamide,or polycarbonate segments. Other surfactants may include a surfactanthaving a block copolymer with one block containing silicon atoms andanother block containing polyether, polyester, polyamde, polycarbonate,polyurethane or polyurea links, or any other surfactant that stabilizesthe small bubbles of gas in the froth that is being produced. The stablefroth produced with the aid of the nucleation surfactants forms easilywith simple agitation schemes and maintains its integrity when putthrough processing equipment with varying temperature, pressures, andshear conditions.

Any suitable gas can be used as the frothing agent. Typically, drynitrogen or dry air are used in the frothing vessel, although carbondioxide, argon or an inert gas (e.g., hydrocarbons, HFCs, HCFCs etc) mayalso be used.

Different cell sizes and different overall densities or porosities canbe achieved by selecting the process temperatures, pressures andagitation schemes. Different pressures can be used at different pointsor times in the process. For example, frothing dispensing, and moldingpressures can all be different. Preferably the stable froth is producedat a temperature of ambient to 100° C. and at a pressure of ambient to100 psig. Preferably the stable froth is metered to the mix head under apressure of ambient to 200 psig.

Various molds or tooling designs may be employed to aid in maintainingor controlling the overall foam density and cell structure of the moldedpart.

Any suitable mixing, foaming, or dispensing equipment is acceptable,including those utilizing recirculation schemes.

An alternate variation of the present invention involves preparing thestable froth in continuous or semi-continuous fashion, in-line between aresin holding tank and the dispensing mix head.

Referring to FIG. 1, a high density foam semiconductor polishing pad orbelt with controlled, reproducible microcellular structure is made byplacing a polymerizable material and a nucleation surfactant in an tank.Although FIG. 1 depicts a single tank, the present invention may bepracticed using two or more tanks or vessels. The polymerizable materialtypically used is polyurethane prepolymer. However, any thermosetablepolymer or prepolymer material could be used including, but not limitedto, a polyamide, a polyester, a polyacrylonitrile, a polyacrylate, apolymethacrylate, a polyvinylchloride, a polyvinyledene fluoride, apolytetrafluoroethylene, a polyethylene, a polypropylene and apolycarbonate. In addition, it is conceivable that all polymerizablematerials including thermoplastics, epoxies, silicone resins and rubberscould be used in this process.

As mentioned above, nucleation surfactants are useful for producing astable froth, which is critical to the present invention. For example ablock copolymer containing at least one block comprisingpolydimethylsiloxane or siloxane polyalkyleneoxide and at least oneother block comprising polyether, polyester, polyamide, or polycarbonatesegments may be used to stabilize the small bubbles of gas that areproduced by the combination of the prepolymer and frothing agent.

The tank 1, which is described in greater detail below in reference toFIG. 2, is typically a cylindrically-shaped vessel with at least oneshaft 2, extending from the upper portion of the tank, to which one ormore impellers 3 are attached. The prepolymer is mixed with thenucleation surfactant by the rotation of the impeller 3 in the presenceof a frothing agent which is metered into the tank by way of a dip tubeor sparge 4 located below the impeller, although alternative embodimentsmay place the sparge at various locations within the tank. In thepreferred embodiment, nitrogen is the preferred frothing agent, althoughit is understood that those skilled in the art may choose to use otherfrothing agents which produce a froth consistent with the presentinvention. The nitrogen gas, metered in from below the impeller, flowsupward, where it is sheared into small bubbles. At steady state, anequal amount of nitrogen breaks the surface and is drawn off from theheadspace of the tank. This continuous flow of nitrogen through the tankremoves volatile components and other gas impurities that are oftenpresent in the froth produced by conventional frothing methods. Thesecontaminants have an adverse impact on the consistency and planarizationperformance of the belts produced from such froth. Consequently, byremoving these contaminants, the present invention results in polishingpads with improved ability to planarize and a more uniform cellstructure which are necessary characteristics of semiconductor polishingpads.

Whereas previous frothing methods only produced froths at pressuresgreater than atmospheric, the present invention produces froth at anyabsolute pressure down to near complete vacuum. The manufacture of frothunder either vacuum or pressure allows for a larger range of specificgravities of the resulting froth, as shown in Example 1 below.

Hence, the froth produced under vacuum will have a preferred densitywhich results in a more uniform cell structure and higher cell densitythan conventional frothing methods. It also provides greater control andflexibility over conventional frothing by allowing the overall densityof the froth to be varied independently from the average cell size. Thisability to vary the density of the froth independently of the averagecell size provides manufacturers a great deal of flexibility to producefroth specifically suited to the individual requirements of certaintypes of industries and applications, all at a lower cost. For example,although specific reference is made to the semiconductor industry in theabove embodiment, the froth of the present invention may be used inother industries such as the pharmaceutical, chemical, and foodindustries.

Referring back to FIG. 1, after the agitation of the froth is completethe impeller is turned off and the inflow of nitrogen is stopped. Thepressure in the tank is increased to a predetermined value and the frothis then metered under pressure, using close tolerance gear pumps, to amix head, where it may be combined with any number of curatives. Thestable froth is metered to the mix head under a pressure of ambient to200 psi. Preferably, the stable froth is metered into the mixhead at apressure of 50 psi.

Prior to entering the mixhead, the curative (e.g., Ethacure 300 fromAlbemarle) is heated and degassed in a separate process tank. Thecurative tank is normally held under vacuum between pours, but will bepressurized prior to dispensing. The pressure in the tank, along withthe close tolerance gear pumps, accurately meters the curative from thecurative tank to the mixhead. Care is taken to keep all the materialsprotected from contact or exposure to moisture. This is accomplished byusing closed tanks and containers and by blanketing with dry nitrogengas instead of ambient room air.

Once the froth and curative are metered into the mixhead, variousadditional components such as fillers, catalysts, additives, blowingagents and surfactants may be incorporated. Catalysts and blowing agentscan be used to create an open-celled structure in the polishing pad orto enlarge the cells after the mixture is poured into the molds. Thefroth, curative and various additives are then thoroughly mixed beforeproceeding through a manifold, where this material is injected into amold cavity, typically at the bottom of the mold. The molds are usuallyopen on top, so that the material overflows the mold at the end of thepour. The material will set-up into a solid in the hot mold in about 5minutes. The casting is removed from the mold and sometimes placed on aretaining ring (to maintain its form) in an oven and fully cured for aprescribed time (typically 16-24 hours for urethanes) prior to beingsent through to secondary machining steps such as turning, grinding,grooving, end-point detection punching, and other trimming andlaminating steps.

Different cell sizes and different overall cell densities or porositiescan be achieved by selecting the process temperatures, pressures, andagitation schemes. Different pressures can be used at different pointsor times in the process. Frothing, dispensing, and molding pressures canall be different. For example, the froth may be produced at pressuresranging from an 18″Hg vacuum to 10 psig. Preferably, the froth isproduced at a vacuum pressure of 4″Hg to 16″Hg. More preferably, thefroth is produced at a vacuum pressure of 6″Hg to 12″Hg. Even morepreferably, the froth is produced at a vacuum pressure of 8″Hg. Anytemperature that result in a froth viscosity of 500-1500 centipoises maybe used. Preferably, the froth is produced at a temperature that resultsin a viscosity of about 1,000 centipoises. More preferably, the stablefroth is produced at a temperature of 150° F.

FIG. 2 depicts a cylindrically shaped tank 1 with a curved base toassist in the mixing of the tank contents. Tank 1, containing prepolymerresin and a nucleation surfactant 7, receives nitrogen from a nitrogensource 6 through dip tube 4. A flowmeter 5 measures the rate of nitrogenflow and control valve 14 may be used to automatically or manuallyadjust the flow rate of the nitrogen entering the tank 1, which isusually set between 1-10 standard cubic feet per hour (scfh) dependingon the size of the tank. Preferably, the flow rate is 5 scfh. One typeof flowmeter commonly used to measure the rate of flow of the incomingnitrogen is a rotometer. The rotometer comprises a vertical tapered tubethat houses a movable float that moves up and down in the tube inproportion to the rate of flow. Rotometer 5 has a direct-reading scalethat is calibrated to read scfh units.

As nitrogen gas enters from dip tube 4 located below the impeller 3 atthe lower portion 13 of the tank 1, an electric motor 8 rotates theshaft 2 and the impeller 3. The motor 8 can be set to rotate the shaft 2and impeller 3 at any speed. Preferably, the impeller rotates at a speedof 400-1100 rpms. More preferably, the impeller rotates at a speed of803 rpms. The impeller of the preferred embodiment consists of ahorizontal disk 10 with vertical blades 11 attached to the perimeter ofthe disc.

The use of high sheer impellers for gas dispersion applications is wellknown in the art. These impellers are typically constructed of metal orplastic. FIG. 3 depicts one common impeller design often referred to asa Rushton impeller, although any other impeller that creates trailingvortices that perform the function of shearing bubbles may be used.Rushton impellers are radial flow, disk turbines, the diameter of whichis typically ⅓ of the tank diameter, and which usually have 4-6 verticalflat blades fixedly attached around the periphery of the disc. TheRushton impeller is designed to provide high shear conditions requiredfor breaking the bubbles of the polymer resin and is often chosen forits ability to deliver high turbulence power numbers. The impellerdepicted in FIG. 3 is of the 4-blade design type 20 and has an overalldiameter of 6 inches. Although FIG. 3 shows a 6″, 4-blade Rushtondesign, it is appreciated by those skilled in the art that othersuitable impellers having different shapes and sizes, different bladecurvatures, different diameters, varying shaft sizes and differentnumber of blades may be used to practice the claimed method depending onvariables such as the gas flow rate and power output of the electricalmotor.

Referring back to FIG. 2, as shaft 2 turns, the fluid 7 inside the tankhits the broad face of the blade and creates a vortex behind it (notshown). This trailing vortex contributes to the creation of the froth bycleaving large bubbles into smaller ones. It is the use of the impeller3 in conjunction with the continuous nitrogen flow coming from below theimpeller that contributes to the froth being produced in a shorterperiod of time and in a more repeatable and efficient manner thantraditional frothing methods.

As illustrated in FIG. 2, the nitrogen is metered into the tank frombelow the impeller and travels upward towards the headspace of the tankwhere it is drawn out of the tank through pressure regulator 16, whichis responsible for accurately controlling the pressure of the tank inconjunction with vacuum pump 15. The nitrogen metered into the tank 1from the lower portion 13, in combination with pressure regulator 16drawing off the nitrogen from the headspace above, allows the tank toreach a steady state of nitrogen flow. This allows a froth to be formedwithout having to create a vortex that draws gas down to the impellerfrom the headspace of the tank. As a result of this efficiency, thefrothing method of the this embodiment is essentially independent of thetank level and results in a more consistent froth, which is asignificant improvement over conventional frothing methods. The presentmethod also allows for very accurate pressure control and is able tocompensate for small gas leaks that caused conventional closed tanks tolose pressure.

Also, as discussed above, manufacturing froth at pressures of slightvacuum to near complete vacuum facilitates improved control over thespecific gravity of the resulting froth, which improves the overalldensity of the polishing pad or belt made from such froth and permitsthe polishing pad or belt to maintain its integrity when it is putthrough processing equipment with varying temperatures and stresses.

The following non-limitative examples illustrate the invention:

Example 1

To demonstrate the superior control over the specific gravity of thefroth of the present invention, froth was prepared at varying vacuumpressures; the temperature and nitrogen inflow remained constant. Thespecific gravity of the froth was measured at vacuum pressures of 18″Hg,12″Hg, 9″Hg, 8″Hg, 4″Hg, and 0″Hg.

63.3 kg of Adiprene® LF 750D (available from Crompton Corporation,Middlebury, Conn.), a polyurethane resin, was added to a controlledprocess tank at a temperature of 150° F. 316 g of polydimethylsiloxanesurfactant (UAX-1600, available from Witco Corporation) was incorporatedinto the Adiprene® resin. The tank agitator was set to a rotationalspeed of 803 rpm using a 6″ 4-blade impeller. Nitrogen gas was meteredin at a rate of 5 scfh through a sparge located at the bottom of thetank. A pressure regulator was used to control the pressure of the tank.

After 2.5 hours of constant agitation at 18″Hg, the impeller wastemporarily turned off, the influx of nitrogen gas was momentarilystopped and tank was brought to atmospheric pressure. A first sample ofthe froth, obtained from the bottom of the tank, was measured to have aspecific gravity of 0.96.

The agitator was once again set to a rotational speed of 803 rpm and thenitrogen gas was metered in at a rate of 5 scfh. The vacuum pressure wasadjusted to 12″Hg. All other variables such as temperature and nitrogenflow remained constant. After one hour of agitation at 12″Hg, theagitator and nitrogen flow were turned off and the tank was brought toatmospheric pressure. A second sample of the froth, obtained from thebottom of the tank, was measured to have a specific gravity of 0.92.

The agitator was set to a rotational speed of 803 rpm and the nitrogengas was metered in at a rate of 5 scfh. The vacuum pressure was adjustedto 9″Hg. All other variables such as temperature and nitrogen flowremained constant. After one hour of agitation at 9″Hg, the agitator andnitrogen flow were turned off and the tank was brought to atmosphericpressure. A third sample of the froth, obtained from the bottom of thetank, was measured to have a specific gravity of 0.87.

The agitator was once again set to a rotational speed of 803 rpm and thenitrogen gas was metered in at a rate of 5 scfh. The vacuum pressure wasadjusted to 8″Hg. All other variables such as temperature and nitrogenflow remained constant. After one hour of agitation at 8″Hg, theagitator and nitrogen flow were turned off and the tank was brought toatmospheric pressure. A fourth sample of the froth, obtained from thebottom of the tank, was measured to have a specific gravity of 0.86.

The agitator was once again turned on and set to a rotational speed of803 rpm and the nitrogen gas was metered in at a rate of 5 scfh. Thevacuum pressure was adjusted to 4″Hg. All other variables such astemperature and nitrogen flow remained constant. After one hour ofagitation at 4″Hg, the agitator and nitrogen flow were turned off andthe tank was brought to atmospheric pressure. A fifth sample of thefroth, obtained from the bottom of the tank, was measured to have aspecific gravity of 0.84.

The agitator was again set to a rotational speed of 803 rpm and thenitrogen gas was metered in at a rate of 5 scfh. The pressure wasadjusted to 0″Hg, which is atmospheric pressure in this example. Allother variables such as temperature and nitrogen flow remained constant.After one hour of agitation at 0″Hg, the agitator and nitrogen flow wereturned off. A sixth sample of the froth, obtained from the bottom of thetank, was measured to have a specific gravity of 0.79.

Graph 1 is diagrammatic illustration of the data obtained from Example 1above. The y-axis represents the specific gravity of the froth in g/cc,and the x-axis is a measure of the vacuum pressure in ″Hg, wherein 30″Hgrepresents complete vacuum and 0″Hg represents atmospheric pressure.Referring to the plotted data of Example 1, it is clear that theproduction of froth under vacuum results in a froth within a range of0.96 to above 0.79. More preferably, Graph 1 illustrates that a frothprepared at a vacuum of 16″Hg to 4″Hg resulted in a froth havingspecific gravities within the range of 0.8-0.9 g/cc, which represents apreferred range of specific gravities for semiconductor polishing beltsand pads. It is only when the pressure of the tank approachedatmospheric pressure and beyond, that the specific gravity of the frothfell out of the preferred range. Consequently, the results of Example 1and the accompanying graph clearly demonstrate that the froth of thepresent invention may be produced within a tight range of specificgravity, in a time-efficient manner.

Example 2

With a 5″ diameter, 4-blade impeller installed, add 60 kg of Adiprene®LF 750D to a temperature controlled process tank. Add 300 g of UAX-1600surfactant. Turn the tank temperature controller on with a setpoint of150° F. Turn on the tank agitator to a speed of 1011 rpm. Turn on thenitrogen sparge to the bottom of the tank with a flow rate of 2 scfh.Vent the headspace of the tank to control at atmospheric pressure. Allowto agitate for at least 4 hours. The resulting froth will haveapproximately a 0.74 specific gravity.

Example 3

-   Batch Preparation: Add 40,000 g of Adiprene® LF 750D from Uniroyal    Chemical Company, urethane prepolymer, to a process tank equipped    with heating and variable speed agitation. Add 2,000 g of UAX-6123,    a nucleating surfactant form Witco Corporation. Pressurize tank with    nitrogen at 7 psig, agitate with simple impeller to create moderate    vortex, and heat to 150° F.-   Dispensing: Stop agitation or resin mixture, and pressurize    Ethacure® 300, diamine curative from Albemarle Corporation, with    nitrogen to 50 psig. Dispense and thoroughly mix resin mixture and    curative simultaneously in a ratio of 102 parts to 21.5 parts by    weight. Adjust backpressure in mixhead to avoid cavitation and to    allow smooth dispensing and expansion of the foam. The resultant    microcellular foam has approximately 0.68 specific gravity.

Example 4

-   Batch Preparation: Add 40,000 g of Adiprene® LF 750D from Uniroyal    Chemical Company, urethane prepolymer, to a process tank equipped    with heating and variable speed agitation. Add 200 g of UAX-6123, a    nucleating surfactant form Witco Corporation. Pressurize tank with    nitrogen at 4 psig, agitate with simple impeller to create moderate    vortex, and heat to 150° F.-   Dispensing: Stop agitation or resin mixture, and pressurize with    nitrogen to 50 psig. In a separate tank, pressurize Ethacure® 300,    diamine curative from Albemarle Corporation, with nitrogen to 50    psig. Dispense and thoroughly mix resin mixture and curative    simultaneously in a ratio of 100.5 parts to 21.5 parts by weight.    Adjust backpressure in mixhead to avoid cavitation and to allow    smooth dispensing and expansion of the foam. The resultant    microcellular foam has approximately 0.92 specific gravity.

Example 5

-   Batch Preparation: Add 25,139 g of Adiprene® LF 750D from Uniroyal    Chemical

Company, urethane prepolymer, to a process tank equipped with heatingand variable speed agitation. Add 1,295 g of UAX-6123, a nucleatingsurfactant form Witco Corporation. Pressurize tank with nitrogen at 50psig, agitate with simple impeller to create moderate vortex, and heatto 150° F.

-   Dispensing: Stop agitation or resin mixture, and dispense foam only    at 50 psig. The resultant uncured microcellular foam is very stable,    with no coalescence or separation. The uncured foam has    approximately 0.41 specific gravity.

While the preferred embodiments of the invention shown and describedabove have proven useful in producing a stable dense froth used tomanufacture pads and belts with controlled, reproducible microcellularstructure, further modifications of the present invention hereindisclosed will occur to persons skilled in the art to which theinvention pertains, and all such modifications are deemed to be withinthe scope and spirit of the present invention defined by the applicableclaims.

1-24. (canceled)
 25. A method of producing a high density, microcellularpad or belt composing the steps of placing a thermosetable liquidpolymer or prepolymer resin and nucleation surfactant in at least onetank, agitating said thermosetable liquid polymer or prepolymer resinand nucleation surfactant through the use of at least one impeller,metering a frothing agent into said tank by way of a tube or spargelocated below said impeller, drawing off said frothing agent from theheadspace of said tank so as to reach a steady state of continuous flowof the frothing agent from below said impeller to said headspace of saidtank, metering the resin froth under pressure to a mix head, combiningthe resin froth with a curative, and injecting or pouring the resinfroth into a mold; said pad or belt being free of hollow microelementsor microballoons.
 26. The method of claim 25, wherein a catalyst isadded to said curative or said froth prior to entering said mold. 27.The method of claim 25, wherein a blowing agent is added to saidcurative or said froth prior to entering said mold.
 28. The method ofclaim 25, wherein said froth is metered into a mixhead at a pressureranging from atmospheric to 200 psi.
 29. A method of making high densityfoam with controlled, reproducible microcellular structure by mechanicalfrothing, comprising agitating a liquid polymer resin in the presence ofa frothing agent and nucleation surfactant at a controlled temperatureand any absolute pressure in order to produce a stable froth, meteringthe resin froth under pressure to a mixhead, optionally combining theresin froth with a curative for the resin, and injecting or pouring theresin froth into a mold.
 30. The method of claim 29, wherein the resinis a thermoset polymer.
 31. The method of claim 29, wherein the resin isa polyurethane.
 32. The method of claim 29, wherein the nucleationsurfactant is a block copolymer containing at least one block comprisingpolydimethylsiloxane, and at least one other block comprising polyether,polyester, polyamide, or polycarbonate segment.
 33. The method of claim29, wherein the nucleation surfactant is a block copolymer containing atleast one block comprising siloxane polyalkyleneoxide, and at least oneblock comprising polyether, polyester, polyamide, or polycarbonatesegment.
 34. A froth for producing microcellular pads and belts withconsistent cell structures or properties, comprising a polymer orthermosetable polymer, a nucleation surfactant, and a frothing agent,wherein said froth is free of hollow polymeric microelements ormicroballoons.
 35. The froth of claim 34 wherein said nucleationsurfactant is a block copolymer containing at least one block comprisingpolydimethylsiloxane or siloxane polyalkyleneoxide, and at least oneother block comprising polyether, polyester, polyamide, orpolycarbonate.
 36. The froth of claim 34, wherein said polymer resin isa polyurethane.
 37. The froth of claim 34, wherein said froth has aspecific gravity within the range of 0.7-1.0 g/cc.
 38. The froth ofclaim 34, wherein said froth has a specific gravity within the range of0.8-0.95 g/cc.
 39. A high density microcellular pad or belt withconsistent cell structures, comprising a polymer or thermosetablepolymer, a nucleation surfactant, and a frothing agent, wherein saidfroth is free of hollow polymeric microelements or microballoons. 40.The pad or belt of claim 39, wherein said nucleation surfactant is ablock copolymer containing at least one block comprisingpolydimethylsiloxane or siloxane polyalkyleneoxide, and at least oneother block comprising polyether, polyester, polyamide, orpolycarbonate.
 41. The pad or belt of claim 39, wherein said polymerresin is a polyurethane.
 42. The pad or belt of claim 39, wherein saidpad or belt has a specific gravity within the range of 0.5-1.2 g/cc. 43.The pad or belt of claim 3, wherein said pad or belt has a specificgravity within the range of 0.7-1.0 g/cc.
 44. The pad or belt of claim39, wherein said pad or belt has a specific gravity within the range of0.85-0.95 g/cc.
 45. The pad or belt of claim 39, wherein said pad orbelt has open-called structures.